1. Field of Invention
The present invention relates generally to a method for forming an interconnected array of semiconductor devices and, more particularly, to a method of making a serially interconnected array of photovoltaic devices.
2. Discussion of the Background
Solar cells represent a source of electrical energy based on an inexhaustable "fuel", with the operation of such devices being nonpolluting. The primary difficulty in the use of a photovoltaic device based electrical energy source has been economics. The costs of fabricating solar cells have heretofore prevented widespread use of such cells for providing electrical energy, and have confined such use to special situations where the fabrication economics do not make their use prohibitive.
A major improvement in economics follows from the use of thin-film semiconductor material in the solar cells rather than crystalline silicon material. Thin-film semiconductive material may be amorphous or polycrystalline in composition. For example, hydrogenated amorphous silicon (a-Si:H), copper indium diselenide (CIS), and cadmium telluride (CdTe) are examples of suitable thin-film materials having a semiconductor direct band gap.
Crystalline silicon is an indirect-band-gap material meaning that a lattice phonon is required to participate in the absorption process with an incident photon. Thus, crystalline silicon absorbs electromagnetic radiation relatively weakly. Thin-film semiconductors of the types mentioned above, on the other hand, are direct-band-gap materials of an effectively larger band gap in which the incident photon can be absorbed without an interaction being required of lattice phonons. As a result, a direct band gap thin-film semiconductor device of a given thickness can absorb as much electromagnetic radiation from the sun as can a crystalline silicon layer many times its thickness typically in a thickness ratio of fifty to one. Thus, very much thinner films of direct band gap semiconductors can be used and still absorb the same amount of incident radiation energy, a structure which reduces the cost of a solar cell considerably.
A-Si:H, CIS, and CdTe are thin-film semiconductor materials which have a relatively well-defined energy gap and in which the semiconductor properties can be controlled by the doping of further impurities. That is, by doping with small amounts of a suitable element, n-type conductivity material and p-type conductivity material can be provided. This situation permits the forming of p-n junction structures or devices or p-i-n structures ("i" meaning intrinsic or near intrinsic semiconductor material) so that direct band gap semiconductor structures subject to incident electromagnetic radiation can be operated as photovoltaic solar cells.
Such solar cells are usually formed in a large array of individual cells to capture large amounts of incident sunlight. For example, A-Si:H is used to make p-i-n homojunction cells. CIS is used with CdS to make CIS/CdS heterojunction cells; and CdTe is used with CdS to make CdTe/CdS heterojunction cells. However, because p-n junctions or p-i-n layer arrangements formed in doped thin-film direct band gap semiconductors yield photovoltaic cells with open circuit voltages measuring several tenths of a volt, there is a desire to electrically interconnect at least some cells in the array in series to provide a greater output voltage. Typically, such cells are formed as a "sandwich-like" structure on a substrate with such cells having, as a general matter, two conductive layers with a semiconductor material layer therebetween where one of the conductive layers is directly on the substrate. The semiconductor layer has a p-n junction or p-i-n layer arrangement more or less parallel to the conductive layers. One of the conductive layers is transparent to pass incident electromagnetic radiation to the semiconductor material layer (the substrate will also be transparent if it directly supports the transparent conductive layer).
There is difficulty with this arrangement in electrically interconnecting the conductive layer adjacent the substrate in one cell, because of it being covered by the other "sandwich" layers, to the conductive layer of a adjacent cell on the opposite side of the semiconductor material layer therein. The series electrical interconnection of cells is especially difficult when the thin-film cell substrate is a conductor such as a metal sheet or foil. In the case of a conducting substrate, the substrate must be cut or slit to form physically separate or discrete cells, the cells arranged on an insulating backing, and the individual cells reconnected in series on the insulating backing.
U.S. Pat. Nos. 4,873,201 and 4,965,655 are good descriptions of the background of the present invention. In the arrangement described by these earlier patents, a laser beam is used to cut a separating space between different parts of the array. An insulator is placed in the space and a penetrating terminal is placed in each part of the array so as to make contact with the lower conductive layer. The top of the penetrating terminal is interconnected to the top of the adjoining portion to form a series interconnection. However, the methods described in these previous patents do not allow monolithic interconnection of solar cells on an electrically conducting substrate. The present invention allows series interconnection of cells on conducting substrate using a roll-to-roll fabrication process.
U.S. Pat. No. 4,746,618 describes a method to slit a web substrate into strips and bond it to an insulating layer. However, the method involves slitting the web transversely to the web length, and necessitates leaving unslit a portion of web near its edge in order to maintain structural integrity using the non-slitted connector. The present invention involves continuous slitting down the length of the web and does not require leaving unslit portions of the web between module segments. Spreader rollers are used instead to control down-web separation of the strips cut lengthwise, not transversely, from a continuously moving web. U.S. Pat. No. 4,746,618 requires step-and-repeat transverse slitting operations.